Filter screens used in the field of this invention are described for example in WO95/23655, WO98/37988 and GB2388558. Typically tensioned wire meshes (wire cloths) are bonded to frames of plastic or metal reinforced plastic materials. The bonding may be by means of an adhesive or by heat softening the plastic surface of the frame and pressing the wire cloths into the softened plastic. Heating the wire cloths may be employed as the method of softening the plastic. After cooling a filtering screen having pretensioned mesh cloths is produced. Preferably two or more wire cloth layers of differing mesh size and wire diameters are employed in a screen. For optimal performance these are pretensioned individually to ensure the correct tension. In some cases they may be pretensioned to different extents.
Careful tensioning of the wire cloths to selected tensions and avoidance of creases or other defects in the tensioned cloths of the filter screens produced is required to avoid loss of screening performance and early failure (holing) of the screens.
Various means of gripping the edges of wire cloths and then applying forces to tension the cloth across a screen frame before bonding thereto are known. Typically a number of clamps are employed to grip the edges of the wire cloth and the clamps are driven (for example pneumatically or mechanically) outwardly of the screen frame to achieve the desired tension. Tension is applied in orthogonal directions to ensure the apertures in the mesh are kept open in the tensioned mesh. The gripping of the cloths should be arranged to allow the tensioning process to occur without slippage of the cloths in the clamps and consequent loss of control of the tension. At the same time the grip should not damage the cloth to the extent that tearing can occur.
It is an object of the present invention to provide methods and apparatus for manufacturing filter screens that avoid or at least minimise one or more of the aforementioned problems.